Communication system



Jan. 1942- R. H. CAMPBELL COMMUNICATION SYSTEM '5 SheetsSheet 1 Filed June 28, 1940 Jan. 6, 1942.

R. H. CAMPBELL COMMUNICATION SYSTEM 3 Shecs-Sheen 2 Filed June 28, 1940 My Qww EN www QN Jan. 6, 1942.

R. H. CAMPBELL COMMUNICATION SYSTEM 3 Sheets-Sheet 5 ifcdmpzezz M 7,

Filed June 28, 1940 Patented Jan. 6, 1942 COMMUNICATION SYSTEM Richard H. Campbell, Racine, Wis, assignor to Webster Electric Company, Racine, Wis, a corporation of Delaware l Application June 28, 1940, Serial No. 342,835

43 Claims.

munication circuits so that no additional inter- 7 station conductors are required.

A further object of the invention is the provision of an arrangement wherein the busy signal is a normally energized device, such as an electric light, whereby the light may be utilized also as a pilot light.

A further object of the invention is the provision of a communication system comprising one or more master stations with a signaling arrangement operable to give an indication at each master station when another station is listening to the conversation thereat; that is, eavesdropping. l

A further object of the invention is to provide a new and improved communication system comprising 'a busy signaling arrangement operative to give a busy signal at the calling stations when several stations simultaneously call a busy station.

A further object of the invention is to prof vide a signaling system having a small power consumption.

' A further object of the present invention is invention will become apparent from the ensuing description, in the course of which reference is had to the accompanying drawings, in which:

Fig. 1 represents schematically one embodiment of the invention;

Fig. 2 represents a modification of the invention; and a Fig. 3 represents a invention. v

In the modification of Fig. 1, there is illustrated an intercommunication system of the loudspeaking type wherein communication is carried on alternately in opposite directions between a plurality of master stations under the control of manually operable communication direction control switches, and wherein communication may be carried on between a master station and one or more substations under the control of a comturther modification or the go' munication direction control switch at the master station.

The master stations are indicated by reference characters I, 2 and 3, and the substation by reference characterl. These stations are provided with translating devices capable of being' respectively.

The amplifiers may be of any .well-knownconstruction and from the figure it may be seen that only'thearnplifier l5 at station I' has been illusto provide a new and improved communication system comprising a busy signaling arrangement wherein undesirable cross talk and hum is prevented.

A further object of the present invention is to provide a communication system comprising master stations having amplifiers and substations having no amplifiers, with combined signal and power control circuits whereby a substation may call a master station and simultaneously operate a signal at the latter and to render operative the amplifier thereat.

A further object of the present invention is to provide a new and improved communication system of the loudspeaking type in which undesirable clicks are obviated during the time that the circuit connections are being changed.

Other objects and advantages of the present trated in detail. In brief. the amplifier comprises an input transformer 24 and an output transformer 25 interconnected by two stages of amplification. Power is supplied to the amplifier through a transformer and in part through a rectifier tube 27. The supply of rectified current (the B supply) is directed to the amplifier through'conductors 28 and 29, the former of which is connected directly to a common conductor 30 of the amplifier which is grounded, as indicated by reference character 3|, and the latter of which. has interposed therein a power switch :2 operable simultaneously with the communication direction control switch l8; The 3 supply circuit to the amplifier is completed when the communication direction control switch s operated to condition the station for either talk-.

ing or listening, and the power is cut of! when the station is in idle condition.

The amplifier is conditioned to be ready for instantaneous use whenever the communication direction control switch is operated simultaneously to condition the station for talking or listening and complete the B supply circuit, by reason of v the fact that the cathode heaters 84 of the two tubes of the amplifier are continuously supplied with heating current from a secondary winding 88 of the transformer 28, the midpoint of which is connected to the common ground conductor 88 by a conductor 88.

The communication direction control switch I8 is a manually operable multi-pole three position switch. In a first position (the up and idle position) the loudspeaker II is connected directly to a communication circuit comprising conductors 48 and 42 for the reception of calls from other stations provided with amplifiers; that is, from other master stations. In a second position (the intermediate and listening position), the loudspeaker is connected to the output and the communication circuit to the input of the amplifier. In a third position (the down and talking position, from which the switch is biased by a The connection of the loudspeaker to the communication circuit in the idle position of the communication direction control switch is as follows: From conductor 48 through switch blade 84 and conductor 48, and from conductor 42 through switch blade 48 and conductor 88. In passing, it may be noted that in the idle position of the communication control switch, the B supply to the amplifier is disconnected by reason of the fact that the circuit through conductor 29 is interrupted by switch blade 82.

The connection of the loudspeaker to the output and the communication circuit to the input of the amplifier in the intermediate position of the communication direction control switch (in which position the switch is illustrated) is effected in the following manner: The loudspeaker II is connected to the output of the amplifier through conductor 48, switch blades 82, 84, conductor 58, a volume control resistor 88, and conductor 88 leading to one terminal of the secondary winding 82 of the output transformer 28, and thence back to the loudspeaker through conductor 84, branch conductor 88, switch blades 88 and I8, and conductor 88. At the same time, resistor I2 is connected in parallel with the loudspeaker II, in order to maintain substantially 1 thence back to the other terminal of the loudspeaker through conductor 84, switch blades I8 and 88, and conductor 88. The communication circuit is connected to the output of the amplifier through switch blade 44, branch conductor 88, and conductor 88 leading to one terminal of the secondary winding 82, and from thence to the other communication conductor 42 through conductor 84, branch conductor 88 and switch blade 48.

In order to avoid undesirable clicking noises while reversing the connections of the loudspeaker and communication circuit to the input and output of the amplifier, there is provided a switch blade 88 and an elongated contact 82 connected, respectively, to the output conductors 84 and 88 so as to short-circuit the output when the communication direction control switch is operated from its listen to talk positions, or vice versa.

One of the important features of the present invention resides in the provision of a busy signal. In the instant embodiment, the signal device is an electric light, the lights at stations I, 2 and 8 being indicated by reference characters 84, 88 and 88, respectively. The light 84 at station I is normally energized whenever the transformer 28 is connected to a source of alternating current, so that the lizht acts also as a pilot light. Control of the energization of the light to perform the signaling function will be described in V greater detail hereinafter.

constant the impedance of the receiving circuit, 1

through a conductor 14 connected to conductor 88 and resistor 88, conductor I8, and switch blade I8. The connection of the communication circuit to the input of the amplifier is efiected through switch blade 44, conductor 88 leading to one terminal of the primary winding 82 of the input transformer 24, and from the other terminal of this primary winding to conductor 42 through conductor 84 and switch blade 48.

The connection of the loudspeaker to the input and the communication circuit to the output of the amplifier in the third position of the communication direction control switch is effected in the following manner: The loudspeaker is connected to the input through conductor 48, switch blades 84 and 82 and conductor 88 leading to one terminal of the primary winding 82, and from I ergy from the rectifier through a circuit including the conductor I88, a resistor H8, and a conductor II2 leading therefrom to the active D01- tion of conductor 28.

The other terminal of the solenoid winding is connected by conductors H4 and H8 to. ground, and the winding is shunted by a condenser I I8.

The control of the solenoid winding is efiected, in part, through a conductor I28 leading from the winding to the communication direction control switch where it is adapted to be engaged by a switch blade I22, when the communication direction control switch is in its idle position. The switch blade I22 is connected by a conductor I24 to a point between resistors I28 and I28 of equal ohmic value connected in series across the communication conductors 48 and 42. The switch blade I22 also has associated with it a grounding circuit I88, which is adapted to be closed whenever the switch blade is in its intermediate or lowermost positions.

By connecting the conductor I24 to a point between the two resistors I28 and I28 (or for that matter, to the center point of a single resistor) a balanced circuit is obtained. The balanced circuit arrangement prevents cross talk and the undesirable hum that would otherwise result from uneven signal voltage (the voltage from the rectifier used in operating the signal means) drops in the communication circuit.

The selector switch 2I at station I includes a number of switches, as; for instance, the two indicated by reference character I82, which are termed the station No. I switches and which are ordinarily maintained in closed position in order that the other stations may communicate with station I. Each of the master stations is provided with as many pairs of switches as there are stations in the system so'that each station may communicate with the others. Thus, the selector switch 22 at station 2 is illustrated as having the same number of switches as selector switch 2I, and the switch blades I34 thereof are closed in order to connect station 2 to the system for the reception of calls from other stations. The selector switch 23 at station 3 has not been illustrated in full and has been shown as including only the two pairs 01' switches I36 and I31,

' the former of which may be considered as being I identically with that at station I, and for this reason it is not deemed necessary to describe the variolm circuit connections in detail. However, the signaling circuits to which the present" invention primarily pertains will be described in order that a better understanding may-be had thereof.

The communication circuit conductors at station 2 are indicated by reference characters I40 and I42. Connected across these in series are the resistors I44 and I46 and from between them there extends a conductor I48 to switch blade I50, which, in the listen position, in which it is indicated, and in the talk position, completes a circuit to ground through thegrounding connection I52. The switch is operable to complete a connection through conductor I54 to the solenoid winding I56. The winding isconnected through conductor I58, resistor I60, and conductor I62 t0 the rectifier in the same manner as the solenoid winding I02 at station I, although lack of space has made it expedient not to illustrate the connections in detail. -The solenoid winding is shunted by a condenser I64 and connected to ground by conductor I66. One terminal of the lamp 96 at station 2 is connected to ground through conductor I68 and the other terminal is connected to a suitable source of potential by a solenoid operated switch I10 and conductor I'I2, which preferably leads to a transformer winding similar to that indicated by reference character- 26 at station I.

The communication circuit at station 3 comprises the conductors I80 and I82 across which are connected in series the resistors I84 and I66 from between which leads a conductor I88 to switch blade I 90. In the indicated listen position of the switch blade and in the talk position, it

completes a circuit to ground through the grounding connection I92. The switch blade is also operable upwardly to complete a circuit through conductor I94 to the solenoid winding I 96, which is supplied with energizing current through a circuit including conductor I98, re

from a suitable alternating current sourcethrough the transformers and that, as a result, the solenoid windings will normally be energized and the electric lights supplied with energy, whereby these lights will act as pilot lights In addition to acting as signaling devices. The

communication direction control switches at the various stations will normally be in their idle positions, at which time the solenoid windings of the signaling systems are connected between the midpoints of resistors. The station selector switches individual to the respective stations are closed so as to condition the stations for the reception of calls from other stations.

Assuming now that the operator at station 2 desires to listen to the conversation at'station I, even though the latter station not be in communication with station 2 or with any other station, then. under these circumstances the signal light 94 at station I will be ale-energized to 'give the operator thereat an indication that someone is eavesdropping. To do this it is necessary forthe operator at station 2 to manipu late one o! the selector switches in order to connect the communication circuit 40-42 to the communication circuit I40I42.

' which position it is illustrated). As a result,

This is done by closing the uppermost pair of switches of selector switch 22 and manipulating the talk-listen switch at station 2 into the listen position (in the point between the resistors I44 and I46 is connected to ground to eii'ect a decrease in the current flowing through the solenoid winding I02, one terminal of which is likewise connected to ground. The resistors are so proportioned that this decrease in current is su-flicient to permit switch I00 to open, thereby to discontinue the supply of power to the light 94.

Satisfactory results have been obtained by connecting the solenoid windings to B supply potentials of approximately 250 volts through resisters (the resistors IIO, I60 and 200) having resistances of 150,000 ohms. The solenoid windings have resistances of about 12,000 ohms and each of the resistors I26, I28, I44, I46, I84 and I86 have resistances of approximately 500 ohms,

The operation of the signal lamp is substantially the same as that described above when station 2 calls station l, the only difierence being that the communication direction control switch at station 2 is depressed into its talking position, at which time, however, the point between the resistors I44 and I46 is grounded through the grounding connection I52 the same as in the listening position of the communication direction control switch. The operator at station I is apprised visually of the fact that someone is either listening or calling station I. In case the station is being called, the call can be heard, of course, through the loudspeaker II thereat.v

The operator at station I responds by talking into loudspeaker II after the operator at station 2 has released communication control switch I9 to its intermediate listen position. Thus, the direction of communication is controlled by the operator at station 2 and the operator at sta-.

tion I does not need to operate the communication control switch IB' thereat. As a matter of fact the operator at station I should not operate switch I3 to its intermediate position when the operator at station 2 is talking, as this would result in double amplification; i. e., the output of amplifier I6 at station 2 would be connected to the input of amplifier I-at station I. If,for

any reason, the operators so desire, communication between the two stations can also be carried on by alternate operation of switches I3 and I9 thereat from their upper idle positions and lower talk positions.

Assuming now that stations I and 2 are communicating with each other according to either of the modes described, then if station 3 selects, for instance, station 2, the signal light 33 at station 3 will be de-energized. When stations I and 2 are in communication with each other, the points between resistors I26 and I23 and between I44 and I43 are grounded through the grounding connections I30 or I52, depending upon the mode of operation and the direction of communication. Then, when the selector switch I31 at station 3 is closed to connect that station to the line individual to station 2, the communication circuit I8II--I'82 of station 3 is placed in parallel with the interstation circuit connecting stations I and 2. As a result, it being assumed that the communication direction control switch at station 3 is in its upper or idle position. then a portion of the current flowing through the solenoid winding I33 is shunted to ground. As explained above, the decrease in current flowing through the solenoid winding is sufilcient to permit opening of switch '2I3, with the result that the lamp 93 is de-energized to give the operator at station 3 an indication that station 2 is busy.

If a number of stations-call stations I and 2 simultaneously when the latter stations are communicating with each other, then the operation of the signal lights at these stations would be seen upon inspection that the amplifiers. the $18- nal devices and control solenoids therefor are identical with those of the previous modification. For this reason it is believed unnecessary substantially the same as that indicated above 1 when station 3 selected station 2.

In case one of the master stations is in communication with the substation 4 and another the fact that a portion of the current flowing through the solenoid I53 will be' shunted to ground. 1

In the modification of Fig. 2, the invention is shown applied to a slightly diflerent type of intercommunicating system. In this system communication between master stations is carried on over two pairs of interstation circuits, one of which is used for communication in one direction and the other for communication in the opposite direction. The modification of Fig. 2 also differs somewhat from that in Fig. 1 in that a slightly different form of communication direction control switch is illustrated. In this case the communication direction control switch is of a type that is adapted to be operated from its intermediate and listen position to a lower and talk position and likewise to an upper and talk position. The switch is biased to return upon release from the lower talk position to the listen position, but remains in the upper talk position when actuated thereinto for the purpose of providing a system adapted particularly for use in installations where the communication may be predominantly in one direction, as for instance between an executive and his secretary.

Referring now in detail to Fig. 2, it may be to describe them in detail and identical reference characters have been applied to indicate these portions of the equipment.

As in the previous modification, the illustrated system comprises four stations, I to 4, inclusive, each of which is provided with loudspeakers II to I4, respectively, and the master stations I, 2 and.) are provided with communication direction control switches 22I, 222 and 223, respectively.

Referring now more particularly to the apparatus at station I, it may be seen that the communication direction control switch thereat is a manually operable three-position switch comprising four switch blades 224, 223, 223 and 233, respectively, all operable simultaneously and biased from the lowermost position to the intermediate position in which it is shown by a spring 232. In the indicated or listen position, the loudspeaker II is connected to an interstation circuit comprising conductors 234 and 233 that is individual to station I. The connection to these conductors is effected through conductor 233, switch blade 223, a potentiometer connection 240, a volume control resistor 242 forming part of the potentiometer connection to one conductor and through conductor 244, switch blade 223 and conductor 243 leading to the other interstation conductor. In this position of the communication direction control switch the B supply to the amplifier is cut off by switch blade 224.

In the lower position of the communication direction control switch the loudspeaker is connected to the primary winding 32 of the input transformer 24 of amplifier I3, through conductors 243 and 233 by means of the switch blades -223 and 223. At the same time the secondary winding 32 of theoutput transformer 23 of the amplifier is connected to another pair of interstation conductors 232 and 234 through con ductor 233 and switch blade 233, and by a conductor 233.

In the upper talk position, the loudspeaker II is connected to the input of the amplifier through the same conductors and switch blades as previously and the output of the amplifier is likewise connected to the interstation conductors 232 and 234 in the same manner.

The output from amplifier I3 is adapted selectively to be connected to the other stations in the system, that is, in the present instance, to stations 2, 3 or 4, through the intermediary of a selector switch, indicated generally by reference character 230, and comprising multi-blade selector switches 232, 234 and 233, respectively. The selector switches serve not only to complete the connection of the interstation circuits, but also serve to complete the signal circuits through switch blades 233, 213 and 2" associated with the selector switches 232, 234 and 233, respectively, in a manner to be described in detail shortly.

switch for selecting substation 4, are connected by a conductor 218 to a point between resistors 288 and 282 of equal ohmic value (to prevent cross talk) connected in series across conductors 234 and 236. These switches serve to ground the point between the resistors through the conductor 218 and grounding connections 264, 288

and 26! associated, respectively, with the selector switches 216, 264 and 266.

The communication direction control switches 222 and 223 at stations 2 and 3 are identical in all respects with the communication direction control switch 22I at station I, both as to the construction of the switch and the circuit conto serve as a busy signal and pilot light and will not give any indication of eavesdropping.

Assumin'g now that station I desires to communicate with station 2, the operator at station Ioperates the selector switch 262 to connect the output of amplifier I6 to the interstationconductors 298 and 292 and simultaneously to ground the point between resistors 288 and 282 through switch blade 268 andground connection 284. When the operator at station 2 responds, he operates the selector switch 384 thereat to connect the output 01 amplifier I6 to thecommunication circuit 234, 238 individual to station I. At the same time, the mid point between resistors. 294 and 298 is. connected to ground through switch blade 388 and ground connection 38I. When both the selector switches have been operated into circuit establishing position, the solenoid windings I82 and I56 are connected to ground through the resistors and communica tion circuits. As a result, the flow 01 current the two there leads a conductor 298 to thev grounding switch blades 388 and 382 associated with selector switches 384 and 386, respectively, by means of which the output of amplifier I6 may be directed to either station I- or station 3, respectively. The ground connection isgeflected throughgrounding connections MI and 383'associated with blades 388 and 382, respectively. The third selector switch for connecting the output of the amplifier to substation 4 ha been omitted for the purpose of clarity and because of space requirements. a

The output of amplifier I6 is led to the selector switches through a pair of conductors 388 and 3" across which is connected in series a pair of resistors 3I2 and 3. The solenoid winding I56 at station 2 is connected to a point intermediate these resistors by a conductor 3I8.

In identical manner the interstation conductors individual to station 3, indicated by reference 7 or station 2. The switches 338 and 332 are through the solenoid windings is decreased and the lights 94 and 96 are both de-energized.

When any other station, such, for instance, as station 3, attempts to call either station I or station 2, when the latter are in communicating relationship with each other, the light 98 thereat is extinguished. This results from the fact that the operation of either selector switch 334 or 336 adapted to complete connections to ground' through the grounding connections 336 and 339, respectively.

The output conductors 348 and 342 have coninoperative but ready for instantaneous use... The solenoid windings I82, I66, and I96 will be energized and power is supplied to the lights 94, 98 and 98 so that these lights act both as signal devices and pilot lig-hts.

In a system of thetype described, a master station cannot listen to another station by reason or the fact that the listening circuit cannot be connected, except to the loudspeaker at .that' station. Therefore, the signal .device functions 75 2 and 3, and the substation by reference char- 7 into circuit establishing position results in connecting the solenoid windingl96 at station 3 to ground through the selector switches at the station selected. Consequently, the operator at station 3 is apprised of the fact that the selected station is busy, whenever the light 98 thereat is" extinguished.

when master station I is paging a substation,

as, for instance, substation 4, and another master station, for instance, master station 2-, selects station I, then the signal-light 96 at station 2 is de-energized to indicate that master station I is busy. This results from the fact that the midpoint between resistors 288 and 282 at station I is grounded through switch blade 2" and ground connection 2II' when station I is placed in communication with station 4 by operation of selector switch 266. Consequently, when selector switch 384 at station 2 is operated to select station I, then the current flowing through solenoid winding I56 is decreased by reason of the fact that part of it is shunted to ground through the resistors and a circuit including interstation conductors 234, 236, conductors 388, 3I8 and theresistors 3I2 and 3I4 at station 2.

The modification of Fig. 3 is in many respects similar to the modification of Fig. 1. It differs from the former in that provision is made for the calling of master stations by the substation and also in that the master stations may call other master stations merely by operation of a push button. In order to eifect these modes of operation, the circuits are so arranged that the substation may render operative the amplifier at the called master station and that a calling master station may render operative the amplifier at a called master station. Use is made of the signal controlling solenoids at the master stations to control the supply of power to the amplifiers and thereby to control the operativeness of the amplifiers.

As in the modification of Fig. 1, the master stations are indicated by reference characters I,

acter 4. These stations are provided with loudspeakers II to I4, respectively, and the master stations are provided with amplifiers I5, I8 and I1, and communication control switches I8, I8 and 520 and station selector switches 2|, 22 and 28, all respectively. The amplifier I8 at station I comprises the input transformer 24 and the output transformer 28 interconnected by two stages of amplification. Power is supplied to the amplifier from a suitable source of alternating current through a transformer 28, and in part through a rectifier tube 21. The supply of rectified current (the B supply) is directed to the amplifier through conductors 28 and 400, the former of which is connected directly to the common conductor 80 of the amplifier, which is grounded through conductor 8i, and the latter of which has interposed therein a power switch 402 operable simultaneously with the communication direction control switch and adapted to connect the B supp to the amplifier when in its lowermost position, that is, when station I is conditioned for talking to other stations. The

as in the modification of Fig. 1.- Thus, loudspeaker II is connected to the output of the amplifier through conductor 48,'switch blades 82 and 84, conductor 88, a volume control resistor 88, and conductor 80 leading to one terminal of the secondary winding 42 of the output transformer 28, and thence back to the loudspeaker through conductor 84, branch conductor 88,

poweris cut 01! in the other positions of the 4 communication control switch, that is, when the switch is in its uppermost position, or in its in-' termediate or idle-listen position. The B supply to the amplifier is adapted to be completed also through a circuit including conductors 404 and 408 and contactor 408, whenever the signal controlling solenoid H0 is de-energized in a manner to be described hereinafter. Whenever the B supply circuit is completed either by operation of the communication direction control switch or the de-energization of the solenoid winding, the amplifier is instantaneously rendered operative by reason of the fact that the cathode heaters 84 of the two tubes of the amplifier are continuously supplied with heating current from the secondary winding 38 of the transformer 28, the midpoint of which is connected to the common ground conductor by a conductor 88.

The communication direction control switch I8 is a manually operable multi-pole three position switch. In a first position (the up position), the loudspeaker II is connected directly to the communication circuit comprising conductors 40 and 42 for calling other stations provided with amplifiers; that is, other master stations. In a second position (the intermediate and idlelisten position), the loudspeaker is connected to the output and the communication circuit to the input of the amplifier for reception from both the other master stations and the substation. In a third position (the down and talking position, from which the switch is biased by a spring 48 into its intermediate position upon release). the loudspeaker is connected to the input and the communication circuit to the output of the amplifier, and the B supply is connected to the amplifier.

The connection of the loudspeaker to the communication circuit in the up position of the communication direction control switch is ei'fected in the same manner as in the modification of Fig. 1, that is, conductor 40 is connected to the loudspeaker through switch blade 44 and conductor 48 and conductor 42 is connected to the loudspeaker through switch blade 48 and conductor 50.

The connection of the loudspeaker to the output and the communication circuit to the input of the, amplifier in the intermediate idle-listen position of the communication direction control switch (in which position the switch is illus-'- trated) is likewise effected in the same manner vductors 88, and ground connection 8|.

switch'blades 88 and I0, and conductor 80. At the same time, resistor I2 is connected in parallel with the loudspeaker H, through the conductor I4 connected to conductor 88 and resistor 88, conductor I8,- and switch blade I8. The connection of the communication circuit to the input of the amplifier is eflected through switch blade 44, conductor 88 leading to one terminal of the primary winding 82 of the input transformer 24, and from the other terminal of this primary winding to conductor 42 through conductor 84 and switch blade 48. I

The connection of the loudspeaker to the input and the communication circuit to the output of the amplifier in the third position of the communication control switch is also'eifected in the same manner as in the modification of Fig. 1. The loudspeaker is thus connected to the input through conductor 48, switch blades 84 and 82 and conductor 80 leading to'one terminal of the secondary winding 82, and from thence back to the other terminal of the loudspeaker r through conductor 84, switch blades 10 and 88, and conductor 50. The communication circuit is connected to the output of the amplifier through switch blade 44, branch conductor 88. and conductor 80 leading to one terminal of the secondary winding 82, and from thence to the other communication conductor 42 through conclgctor 84, branch conductor 88 and switch blade In order to avoid undesirable clicking noises while reversing the connections of the loudspeaker and communication circuit to the input and output of the amplifier, there is provided the switch blade 80 and an elongated contact 82 connected, respectively, to the output conductors 84 and 80 so as to short-circuit the output when the communication direction control switch is operated from its intermediate to lowermost positions, or vice versa.

In the instance embodiment, the signal devices at stations I, 2 and 8 are again indicated by reference characters 84, 88 and 88. The'light 84 at station I is normally energized whenever the transformer 28 is connected to a source of alternating'current, so that the light acts also as a pilot light. Control of the energization of the light to perform the signaling function will be described in greater detail hereinafter.

The supply of energy to the lamp is controlled by the contactor 488 operated by the normally energized solenoid winding 4". When the solenoid winding is energized, the light 84 is connected across a portion of the secondary winding 88 of transformer 28 through a circuit including the ground connection I04 and the conductor I08 leading to one terminal of the transformer-the circuit being completed through con- The solenoid winding 4I8 is normally supplied with energy from the rectifier through a circuit including conductors 404, 4I2, resistor H4, and conductors H8 and 4I8.

The other terminal of the solenoid winding is connected to ground through conductors 428 and 422, -and the winding is shunted by a conpoint intermediate the resistors I28 and I28 connected in series across the interstation conductors 40 and 42. The point intermediate the resistors isalso connected by a conductor 428' to switch blade 428 operable simultaneously with the communication control switch. The switch blade is effective to ground the midpoint through a conductor 430 in the lowermost position of the communication direction control switch and to condition that point for grounding by a push button switch 432 utilized in calling other master stations and conductors 484 and 488 in the uppermost position of the switch. w

The selector switch 2I at station I includes a number of switches, as, for instance, the two indicated by reference character I82, which are termed the station No. I switches and which are ordinarily maintained in closed position in order that the other stations may communicate with station I. Each of the master stations is provided with as many pairs of switches as there are stations in the system so that each station may communicate with the others. Thus, the selector switch 22 at station 2 is illustrated as having the same number of switches as selector switch 2I, and the switch blades I84 thereof are closed in order to connect station z-w the system for the reception of calls from the other stations. The selector switch 23 at station 8 has not been illustrated in full and has been shown as including four pairs of switches, of.

which switches I88 may be considered as being the switch associated with station 8 v r The loudspeaker I4 at station 4 is connected by conductors 440 to a selector switch 442 of the rotary type and adapted to be operated into a number of positions equal to the number of stations and illustrated as beingconnected to the lowermost pair of interstation conductors. Conpletes an energizing circuit for the light 88 through conductor 418 and ground connection 480. When the solenoid is de-energized, the B supply to amplifier I8 is completed through conductors 482 and 484.

The communication circuit at station 8 comprises the conductors I80 and I82 across which are connected in series the resistors I84 and I88, from between which leads a conductor 488 to switch blade 488. Switch blade 488 is adapted in.

its lowermost position to connectconductor 488 to ground through conductor 480, and in its uppermost position to condition the connection of conductor 488 to ground through conductor 482, push button switch 484 and conductor 488.- The point intermediate vresistors I84 and I88 is connected to solenoid winding 408 byconductor 488 .and a conductor 800. The solenoid winding is adapted normally to be energized with current supplied thereto from the rectifier associated with the amplifier II, through a circuit including resistor 802 in the same manner as the solenoid windings at the other master stations. The solenoid winding is shunted by condenser 80.4 and connected to ground through a conductor 808. The solenoid winding is normally energized and when energized, the contactor 808 associated therewith completes an energizing circuit for light 88 through conductors 8I0 and 8I2. When de-energized, the contactor 808 completes the connection of the B supply to amplifier I! through conductors 8 and 8I8L In operation, the communication direction control switches are left in the positions indicated, i. e., in their idle-listen positions. wherein the interstation circuits are connected to the inputs of the amplifiers and the loudspeakers are connected to the outputs. Assuming that one of the are operated to connect the interstation conducductors 440 are interconnected by the series con- I nected resistors 444 and 448 of equal ohmic value,

and the pointintermediate the two is adapted to be connected to ground by a push button switch 448 and conductors 480 and 482, for purposes to be described in greater detail hereinafter.

The apparatus at stations 2 and 8 corresponds identically with that at station I and will, therefore, be described onLv briefly.

- push button switch 482 and conductor 484. The

point intermediate resistors I84 and I88 is connected by conductors 484 and 488 to solenoid winding 488, which winding is adapted to be connected to the rectifier associatedwith amplifier I8 through the resistor 410 in a manner identical with the connection of solenoid H0 at station I, although this connection has not been illustrated fully. The solenoid winding is shunted by a conmaster stations, for instance, master station 2, desires to communicate with master station I, then the uppermost pair of switches at station 2 tors I48 and I42 of station 2 to the interstation conductors 40 and 42 of station I. Operation of the selector switch in this manner does not affect the energization of solenoid H0 or 488, because these are connected to points of equal potential uppermost position and depressing push button switch 482. If this mode of operation is followed, then the loudspeaker I2 at station 2 is connected to the input of amplifier I8 at station I, and the output of that amplifier is connectedto loudspeaker II. The amplifier I8 is rendered operdenser 412 and one terminal thereof is connected to ground through conductor 414. The sole- 4 noid winding is normally energized and the switch 418 associated therewith normally comative by reason of the grounding of solenoid 4I0 I given a busy signal if these stations are in communication with each other, immediately upon operation of the selector switch thereat. In' this event, the solenoid winding 488 at station 8 is the push button switch 462, or this switch and switch 428.

A substation is able to call a master station whenever the communication control switches at the master station are in their intermediate idlelisten positions. For instance, substation4may be placed in communication with master station I by operation of selector switch 442 to connect conductors 440 to conductors 40 and 42 through the normally closed selector switch blades I32 at station I., If this is done, loudspeaker I4 at station 4 is connected to the input or amplifier It, the output 01' which, is connected, as previously described. to loudspeaker II at station I. Then, upon closure or push button switch 448 at station 4, amplifier ll is rendered operative by reason or the tact thatsolenoid 4ll is grounded through a circuit including the interstation conductors, the resistors 444, 446, and conductors 440, and 452. The operator at station I may thereafter control the direction of communication as long as push button switch 448 at station 4 is maintained in its closed position.

When stations I and 4 are in communication with each other, 'then, it station 2 desires to communicate with station I. the signal light 98 at station 2 is de-energized as soon as station I is selected by operation or the proper selector switch. The reason for this is, as previously explained, that solenoid 488 is connected to ground through the interstation circuits and through either switch blade 428 at station I or push button switch 448 at station 4, or both.

It is to be noted that while only a comparatively few stations have been shown in each or the embodiments oi the invention illustrated, it should be understood that the number or stations may be multiplied to any extent desired.

Although the invention has been disclosed with the specific details of preferred embodiments thereof, it should be understood that such details are not intended to be limitative oi the invention, except in so far as set forth in the accompanying claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. In a multiple station intercommunicating system, the combination including, signal means at each station, current responsive means normaliy occupying a predetermined position controlling the signal means at each station, a station selector-switch at each station, and means operable by the station selector switch at each station for changing the position of the current responsive means thereat upon selection of a station already in communication with another station to indicate whether or not the selected station is busy.

2. In a multiple station intercommunicating system, the combination including, signal means at each station, a source oi energy at each station, and means including a solenoid normally energized from said source and a station selector switch at each station for indicating whether or not the selected station is busy.

3. In a multiple station intercommunicating system, the combination including, signal means at each station, a source of energy at each station, a solenoid normally energized from said source for controlling the signal means, and means including a station selector switch for shunting said solenoid to deenergize the latter upon establishment of communication circuits with another station.

4. In a multiple station intercommunicating system of the loudspeaking type, the combination including, an amplifier, signal means, signal control means normally energized from the amplifier,

communication control switch and station selector switches at each station, and interstation circuits, said signal control means at each station being so connected in circuit that they are grounded when the stations are in communication with other stations, whereby when another station selects a station already in communication with another the signal controlling means thereat is grounded to operate the signal to indicate that the selected station is busy.

5. A unit for use in a multiple station intercommunicating system, including in combination, signal means, current responsive means normally occupying a predetermined position controlling or not the selected station is busy.

'7. A unit for use in a multiple station intercommunicating system, including in combination, signal means, a source of energy, a solenoid normally energized from said source for controlling the signal means, and means including a station selector switch for shunting said solenoid to de-energize the same upon establishment or communication circuits with another station. 8. A unit for use in a multiple station intercommunicating system of the loudspeaking type, including in combination, an amplifier, signal means, signal control means normally energized from the amplifier power supply, a communication control switch, and station selector switches,

said signal control means being so connected in circuit that it is grounded when the station is in communication with another station, whereby when the station selects a station already in com.-

munication with another the signal controllingmeans is grounded to operate the signal to indicate that the selected station is busy.

9. In a multiple station intercommunicating system of the type wherein each station comprises an amplifier, a loudspeaker adapted to be utilized bothas a microphone and a receiver, a communication control switch operable in one position to connect the station for reception of calls from other stations, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches, the combination including, signal means, signal controlling means, means normally supplying said controlling means with sufilcient energy to operate it into a predetermined position, and switch means operable by said communioation control switch when the latter is operated to said one position to connect said signal controlling means to said pair of wires for control by other stations.

10. A unit for use in a multiple station inter- I communicating system wherein each unit com- 9,969,141 position to connect the station for reception of I control by other units.

11, In a multiple station intercommunicating system of the type wherein each station comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in one position to connect the station for reception'ot calls directly from other stationsand in a second.

position for reception of calls from other stations through the amplifier, station selector'- switches, and a single pair 01 wires leading from the communication control switch to the selector switches, the combination including, signal means, signal controlling means, means normally supplying saidcontrolling means with sufiicient energy to operate it into a predetermined position, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to said one position to connect said signal controlling means to a point between said resistors and to connect said point to ground in said second position, ,whereby when the communication control switch at one station is in said one position and another station selects that station and operates the communication control switch thereat to its second position the signal controlling means is grounded to operate the signal to give an indication that another station is listening to the one station.

12. A unit for use in a multiple station intercommunicating system of the type wherein each unit comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in one position to connect the unit for reception of calls directly from other units, and in a second position for reception oi calls from other units through the amplifier, unit selector switches, and a single pair of wires leading from the communication control switch to the selector switches, including in combination, signal means, signal controlling means, means normally supplying said controlling means with sufllcient energy to operate it into a predetermined position, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to said one position to connect said signal controlling means to a point between said resistors and to connect said point to ground in said second position, whereby when the communication control switch of the unit is in said one position and another similar unit selects that unit and operates the communication control switch thereat to its second position thev signal controlling means ls grounded to operate the signal to give an indication that another unit is listening to said unit.

13. In a multiple, station intercommunicating system of the type wherein each station comprises an amplifier, a loudspeaker adapted to be utilized both as amicrophone and a receiver, a 75 I normal position to connect the station for reception of calls directly from other stations, in a second position for reception oi! calls from other stations through the amplifier, and in a third position to\ transmit calls to through the amplifier, station selector switches, and a single pair of wires leading from the communication control switch to the selector switches the combination including, signal means, signal controlling means, means normally supplying said controlling means with sufiicient energy to operate it into a predetermined position, a

pair of resistors connected across said wires,;and switch means operable by said communication control switch when the latter is operated to its normal position to connect said signal control-- ling means to a point between said resistors and to connect said point to ground in said second and third positions, whereby when the communication control switch at one station is in its normal position and another station selects that station and operates the communication control switch thereat to its second position, the signal controlling means is grounded to operate the signal to give an indication that another station is listening to the one station, and when a third station selects one of two stations communicating with each other by alternate operation of the communication control switches thereat between the first and third positions when the communication control switch at the third station is in its normal position, the signal controlling means at the third station is grounded to indicate that the selected station is busy.

14. A unit for use in a multiple station intercommunicating system of the type wherein each unit comprises an amplifier, a loudspeaker adapted to be utilized both as a microphone and a receiver, a communication control switch operable in its normal position to connect the unit for reception of calls directly from other units, in a second position for reception of calls from other units through the amplifier, and in a third position to transmit calls to other units. unit selector switches, and a single pair of wires leading from the communication control switch to the selector switches, the combination including, signal means, signal controlling means, meansnormally supplying said controlling means with sufiicient'energy to operate it into a predetermined position, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to its normal position to connect said signal controlling means to a point between said resistors and to connect said point to ground in said second and third positions, whereby when the communication control switch at one unit is in its normal position and another unit selects .control switches between their first and third positions when the communication control switch at the unit is in its normal position, the signal controlling means at the unit is grounded to indicate that the selected unit is busy.

15. In a multiple station intercommunicating system of the type wherein first and second stations comprise an amplifier, a loudspeaker communication control switch operable in its other stations adapted to be utilized both as a microphone and a receiver, a communication control switch operable in its normal position to connect the station for reception of calls directly from other stations, in a second position for reception of calls mined position. a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to itsrnormal position to connect said signal controlling means to a point between said resistors and to connect said point to ground in said second and third positions at said. first and second stations, whereby when either said first or second station is communicating with the third station by operation oi the communication control switch thereat to its second and third positions ond the other of said stations selects either of the stations communieating with each other with its communication control switch in its normal position, the signal control means at the other station is grounded to indicate that the selected station is busy.

16. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, and selector switches adapted to connect the outgoing line to other stations, the combination including, signal means, signal controlling means, means normally supplying said controlling means with sufficient energy to operate it to a predetermined position, and switch means operable with the selector switches so constructed and arranged that when two stations are connected for communication with each other the signal controlling means are grounded.

17. A unit for use in a multiple station intercommunicating system, including in combination, an incoming line, an outgoing line, signal means, signal controlling means connected to said outgoing line. means normally supplying said controlling means with sufilcient energy to operate it to a predetermined position, and switch means in said outgoing line for grounding the incoming line when the unit is conditioned for communication with another unit.

18. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, and selector switches adapted to connect the outgoing line to other stations, the combination including, signal means,, signal controlling means, means normally supplying said controlling means with sufficient energy to operate it into predetermined position, a pair of resistors connected across the outgoing line, a connection from between said resistors to said signal controlling means, a pair of resistors connected across the incoming line, and means including switches operable with the selector switches for grounding a point between said last-mentioned resistors whenever aselector switch is operated.

19. A unit for use in a multiple station intercommunicating system, including in combination, an incoming line, an outgoing line,

signal means, signal controlling means, means normally supplying said controlling means with suflicient energy to operate it into a predetermined position, a pair of resistors connected across the outgoing line, a connection from between said resistors to said signal controlling means, a pair of resistors connected across the incoming line, and means including a switch operable with the selector switch for grounding a point between said last-mentioned resistors whenever the selector switch is operated.

20. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, selector switches adapted to connect the outgoing line to other stations, a loudspeaker adapted to be utilized both as a microphone and a receiver, an amplifier, and a communication control switch operable in one position to connect the loudspeaker to the incoming line for the reception of calls from other stations, the combination including, signal means, signal controlling means, means normally supplying said controlling means with sufficient energy to operate it into a predetermined position, and switch means operable with the selector switches so constructed and arranged that when two stations are connected for communication with each other the signal controlling means thereat are grounded.

21. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, selector switches in the outgoing line to connect the line to other stations, a loudspeaker adapted to be utilized both as a microphone and a receiver, an amplifier, and a communication control switch operable in one position to connect the loudspeaker to the incoming line for reception of calls from other stations, the combination including, signal means, signal controlling means, means normally supplying said controlling means with sumcient energy to operate it into a predetermined position, a pair of resistors connected across the outgoing line, a connection from between said resistors to said signal controlling means, a pair of resistors connected across the incoming line, and means including a switch operable with the selector switch for grounding a point between said last-mentioned resistors whenever the selector switch is operated to select a station.

22. In a multiple station intercommunicating system of the type wherein each station comprises an incoming line, an outgoing line, selector switches in the outgoing line for connecting it to other stations, a loudspeaker adapted to be utilized both as a microphone and a receiver, an amplifier, and a communication control switch operable in one position to connect the loudspeaker to the incoming line for reception of calls from other stations, and in another position to connect the loudspeaker to the input of the amplifler and the outgoing line to the output of the amplifier for the transmission of calls to the other stations, the combination including, signal means, signal controlling means, means normally supplying said controlling means with suificient energy to operate it into a predetermined position, a pair of resistors connected across the outgoing line, a connection from between said resistors to the signal controlling means, a pair of resistors connected across the incoming line, switches operable simultaneously with the selector switches for grounding the midpoint between the resistors connected across the incoming line, whereby when two stations are placed in communication with each other the signal controlling means-at said stations are grounded to operate the signal means, and when a selector switch of a third station is operated to select a busy station the signal controlling means at the third station is grounded to operate the signal means. I

23. A unit for use in a multiple station intercommunicating system, including in combination, an incoming line, an outgoing line, signal means, signal controlling means, means normally supplying said controlling means with sumcient energy to operate it into a predetermined position, a pair of resistors connected across the 'outgoing line, the midpoint of. which is connected to the signal controlling means, a pair of resistors connected across the incoming line, unit selector switches in the outgoing line, switches operable simultaneously with the selector switches'i'or grounding a point between the last-mentioned resistors, whereby the signal controlling means are grounded when two units are placed in communication with each other and when a busy 'unit is selected to operate the signal means.

24. In a multiple station intercommunicating system of the type wherein a station comprises an incoming line, an outgoing line, selector switches in the outgoing line to connect the line to other stations, a loudspeaker adapted. to be utilized both as a microphone and a receiver, an amplifier, and a communication controlswitch operable in one position to connect the loud-.

speaker to the incoming line for reception of calls.

from similar stations and into a second position for connecting the loudspeaker to the input of the amplifier and the output of the amplifier to the outgoing line, and wherein another station comprises a loudspeaker utilizable both as a microphone and a receiver, the combination including, signal means, signal controlling means, means normally supplying said controlling means with sufiicient energy to operate it into a prede termined position, a pair of resistors connected across the outgoing line, a connection from a 7 stations similar to the first are grounded to operate the signal means upon selection of the first station.

25. In a multiple stationintercommunicating system of the type whereineach station comprises an amplifier, a loudspeaker adapted to be utilized bothas a microphone and a receiver, a communication control switch operable in one position to connect the station for reception of calls from other stations and operable in a second position to connect the station for transmission of calls to other stations, station selector switches, and a single pair of' wires leading from the communication control switch to the selector switches, the combination including, means, signal controlling means, means normally supplying said controlling means with sufiicient energy to operate it intoa predetermined position, a pair of resistors connected across said wires, a connection from the signal controlling means to a point between said resistors, and switch means operable with the communication control switch for grounding said connection when the latter is operated to its second position.

signal 26. A unit for use in a multiple station intercommunicating system, including in combination, signal means, signal controlling means, means normally supplying said controlling means with suflicient energy to operate it into a predetermined position, a communication circuit, a pair of resistors connected across said circuit, and a connection from the signal controlling means to a point between said resistors. l

27. In a multiple station intercommunicating system, the combination including, an amplifier at each station, signal means at each station, a source of energy at each station, a communication circuit and station selecting means at each station, and means including a solenoid normally energized from said source, a pair of resistors connected across the circuit, and means including a grounding switch grounding the communication circuit through the resistors at each station and deenergizing the solenoid for indicating whether or not the selected station is busy and for rendering operative the amplifier at .the selected station.

28. A unit for use in a multiple station intercommunicating system, including in combination, an amplifier, signal means, a source of energy, a solenoid normally energized from said source, a communication circuit, a pair of resistors connected across said circuit, a connection from v a pair of resistors connected across said con ductors, a connection from between said resistors to said controlling means, means normally supplying said controlling means with energy suiiicient to operate the same to a predetermined position, and switchmeans associated with said communication .control switch for grounding the point between said resistors upon operation of the communication control switch to a position to condition the station for transmitting calls to other stations, whereby when a station selects a busy station the signal controlling means at theselecting station is grounded to operate the busy signal thereat.

30. In a multiple station intercommunicating system,of the type wherein each station comprises an amplifier, a loudspeaker adapted to be utilized both. as a microphone and a receiver, a communication control switch operable in its normal position to connect the station for reception of calls directly from other stations, in a second position for reception of calls from other stations through the amplifier, and in a third position to -transmit calls to other stations through the amplifier, station selector switches, 65 and a single pair of wires leading from the com- 'munication control switch to the selector .switches, the combination including, signal means, signal controlling means, means normally supplying said controlling means with sufiicient energy to operate it into a predetermined position, a pair of resistors connected across said wires, and switch means operable by said communication control switch when the latter is operated to its normal position to connect said signal controllinglmeans to a point between said eration of the communication control switch atone station between its second and third positions when the communication control switch at the third station is in its normal position, the

signal controlling means at the third station is grounded to indicate that the selected station is busy.

31. In an intercommunicating system of the type comprising a plurality of stations, each provided with an amplifier, a loudspeaker adapted to function both as a microphone and a receiver, a pair of conductors over which communication may be carried on in opposite directions under the control of a communication control switch adapted in a normal position to connect the loudspeaker to the output and the conductors to the input of the amplifier, in a second position to reverse the connections and render operative the amplifier, and in a third position .to connect the loudspeaker directly to the conductors and station selector switches, the combination including, signal means, signal and amplifier controlling means, a pair of resistors connected across said conductors, a connection from between said resistors to said controlling means, means normally supplying said controlling means with energy sufiicient to operate the same to a predetermined position, switch means associated with said communication control switch for grounding the point between said resistors upon operation of the communication control switch to its second position and for conditioning the connection of said midpoint to ground when operated into its third position, a normally open push button switch for completing the connection to ground when the communication control switch is in its third position, whereby one station can call another by operation of the communication control switch thereat to its third position and closing the push button switch, and communication may be carried on between the calling and called stations by manipulation of the communication control switch at the called station between its normal and second positions while the push button switch at the calling station is maintained closed, and whereby when a third station selects either of the two stations in communication with each other the signal controlling means thereat is grounded to operate the signal means to give an indication that the selected station is busy. 1

32. In an intercommunicating system of the type comprising a station provided with an amplifier, a loudspeaker adapted to function both as a microphone and a receiver, a pair of conductors over which communication may be carried on in opposite directions under the control of a communication control switch adapted in a normal position to connect the loudspeaker to the output and the conductors to the input of the amplifier, and in another position to reverse the connections and render operative the amplifier, and a second station comprising a loudspeaker adapted to function both as a microphone and a receiver, the combination including, signal means, signal and amplifier controlling means, a pair of, resistors connected across said conductors, a connection from between said resistors to said controlling means, means normally supplying said controlling means with energy sufiicient to operate the same to a predetermined position, and switch means associated with said communication control switch for grounding a point between said resistors upon operation of the communication control switch to its second position to condition the first station for transmitting calls to other stations, a pair of conductorsleading tothe second station, a pair of resistors connected in series thereacross and means including a normally open push button switch adapted for grounding a point between said resistors, whereby when the communication control switch at said one station is in its normal position the second station may call the first station by operation of the push button switch to ground the signal and amplifier controlling means at the first station thereby to render the amplifier operative, and whereby communication may be carried on between said first and second stations by operation of the communication control switch at the first station between its, normal and second positions as long as the push button switch at the second station is maintained closed.

33. In an intercommunicatlng system of the,

type comprising a plurality of stations each provided with an amplifier, a loudspeaker adapted to function both as a microphone and a receiver, a pair of conductors over which communication may be carried on in'opposite directions under the control of a communication control switch adapted in a normal position to connect the loudspeaker to the output and the conductors to the input of the amplifier, and in another position to reverse the connections and render operative the amplifier, and another station comprising a loudspeaker adapted to function both as a microphone and a receiver, the combination including, signal means, signal and amplifier controlling means, a pair of resistors connected across said conductors, a connection from between said resistors to said controlling means, means normally supplying said controlling means with energy suflicient to operate the same to a predetermined position, and switch means associated with said communication control switch for grounding the point between said re-,

sistors upon operation of the communication control switch to its second position to condition any of said plurality of stations for transmitting calls to other stations, selector switch means, a pair of conductors leading to the other stations, a pair of resistors connected in series thereacross and means including a push button switch adapted to ground a point between said resistors, whereby when the communication control switch at one of said plurality of stations is in its normal position others of said plurality of stations 39. In a multiple station intercommunlcating erated by the grounding of the signal controlling means upon selection of said one station;

34. A nit for use in an intercommunicating system, including incombination, a communication circuit, an amplifier, a multi-position communication and amplifier control switch, a solenoid and a switch operable thereby between two circuit closing positions, a signal light adapted to be energized when said switch is in one circuit 7 closing position, means to energize the amplifier when said switch is in the' other of itscircuit closing positions, a pair of resistors connected across said circuit, a connection from between said resistors to said solenoid, means normally supplying said solenoid with energy sufficient to operate it to a position in which the switch associated therewith is in said one circuit closing position, switch means associated with said communication control switch for grounding a point between said resistors upon operation of the communication control switch to a position wherein the amplifier control means associated with said switch is in a position to render the amplifier operative, and for conditioning the connection of said point to ground when operated to a position wherein the amplifier is rendered inoperative, and a normally open push button switch for completing the connection to ground when the communication control switch is in said last mentioned position.

35. An intercommunicating system comprising a plurality of stations, each having a source of power, a signal lamp, a normally operative signal lamp controlling means adapted-to connect the lamp to said source of power, whereby said lamp serves as a pilot light, station selector switches, and means for rendering said controlling means inoperative to connect said lamp to said source when a busy station is selected, whereby said lamp serves also as a busy signal.

36. In a unit for use in an intercommunicating system, the combination including, an amplifier, a loudspeaker adapted to serve both as a microphone and a receiver, a communication direction control switch for controlling the connection of said speaker to the'input and output of said amplifier, and means operable simultaneously with said switch for short-circuiting the output of the amplifier while the connections of said speaker to the amplifier are being changed. 37. In a multiple station intercommunicating system, the combination including, a first station including a loudspeaker adapted to be utilized as a microphone connected to an interstation communication circuit and signal means, a second station including an amplifier having input and output connections, a loudspeaker and a communication control switch operable to a position wherein said communication circuit is connected to the input and the loudspeaker to the output of said amplifier, and means operable by said control switch to operate the signal system, the combination including, an amplifier at each station, a source 01 energy at each station, a communication circuit and station. selecting means at each station, and means including a solenoid normally energized from said source, a pair of resistors connected .across the circuit, and means including a grounding switch grounding the communication circuit through the resistors at each station and deenergizing the solenoid for rendering operative the amplifier at the selected station.

40. A unit for use in a multiple station intercommunicating system, including in combinameans at the first station when said switch is tion, an amplifier, a source of energy therefor, a solenoid normally energized from said source, a communication circuit, a pair of resistors connected across said circuit, a connection from said solenoid to a pointbetween said resistors, and switch means'operated by said solenoid for controlling the connection of said amplifier to said source of energy.

41. In an inte'rcommunicating system ofthe type comprising a station provided with an amplifier, a loudspeaker adapted to function both as a microphone and a receiver, a pair of conductors over which communication may be carried on in opposite directions under the control of a communication control switch adapted in a normal position to connect the loudspeaker to the output and the conductors to the input of the amplifier, and ,in another position to reverse the connections and render operative the amplifier, and a second station comprising a loudspeaker adapted to function both as a microphone and a receiver, the combination including, amplifier controlling means, a pair of resistors connected across said conductors, a connection from between said resistors to said controlling means, means normally supplying said controlling means with energy sufiicient to operate the same to a predetermined position, and switch means associated with, saidcommunication control switch for grounding a point between said resistors upon operation of the communication control switch to its second position to condition the first station for transmitting calls to other stations, a pair of conductors leading to the second station, a pair of resistors connected in series thereacross and means including a normally open push button switch adapted for grounding a point between said resistors, whereby when the communication control switch at said one station is in its normal position the second station may call the first station by operation of the push button switch to ground the amplifier.controlling means at the first station thereby to render the amplifier operative, and whereby communication may be carried on between said first and second stations by operation of the communication control switch at the first station between its normal and second positions as long as the push button switch at the second station is maintained closed.

42. A unit for use in an intercommunicating system, including in combination, a communication circuit, an amplifier, a multi-position communication and amplifier control switch, a solenoid and a switch operable thereby into a predetermined position for energizing the amplifier, a pair of resistors connected across said circuit, a connection from between said resistors to a position wherein the amplifier control means 5 associated with said switch is in a position to render the amplifier operative, and for conditioning the connection 0! said point to ground when operated to a position wherein the amplition to ground when the communication control switchis in said last mentioned position.

43. ln gan intercommunicating system, the combination including, a pair of stations adapted to be connected by a communication circuit, an amplifier at one station, a solenoid controlling the operation of said amplifier, and means at the other station for controlling said solenoid over said communication circuit to energize said i'ier is rendered inoperative, and a normally open 10 amplifier.

push button switch for completing the connec- RICHARD H. CAMPBELL. 

